Daylight harvesting is an available lighting strategy designed to reduce excessive internal light levels during peak consumption hours, wherein external light sources, such as daylight, substitute for interior electrical lighting. For example, in an office setting, each work area must at all times be provided with a minimum level of light which is determined based upon the tasks performed in the area or zone. Lighting, however, is generally installed by size and number sufficient to provide the minimum light level under the assumption that no other light sources are available in the interior space. Yet, during varying times of the day, other light sources may illuminate the interior space such that the level of light present is excessive. Thereby, the use of interior lighting at the same level of intensity becomes a waste of energy.
Specifically, during the day, sunlight may enter through windows and skylights. When these external light sources are present, the preset brightness of interior lighting is not necessary since these external light sources provide some or all of the minimum light level required. Daylight harvesting eliminates the excessive level of intensity of interior lighting, conserving as much as 84% of the energy required to light a facility at the minimum light level. As such, during midday, excess electrical lighting is minimized and bright sunlight is utilized to provide up to 100% of illumination during midday, when energy costs are highest.
Daylight harvesting also provides a constant level of light on work surfaces to avoid moments when the external light sources provide an excessive amount of light, resulting in periods of glare. In the alternative, when light levels are low (i.e. when clouds roll in or nighttime falls), daylight harvesting maintains this constant level of light by continuously increasing and decreasing the power applied to the internal lighting. This practice enables the worker to resolve images with ease. As a result, eyestrain is avoided; and health and productivity are promoted.
Conventional technology for implementing daylight harvesting techniques incorporates the use of digital photo-sensors to detect light levels and dimmers to automatically adjust the output level of electric lighting for promoting balance. Dimming control circuits, as implemented with respect to daylight harvesting, gradually increase or decrease interior lighting in response to photocell measurement of ambient light levels.
There are two kinds of light sensors are available. The “open-loop” sensor is positioned within a lighting system such that the sensor monitors the amount of light outside of a nearby window or skylight to read only the amount of light coming into the interior space from outside. The open loop sensor may be located within the interior space or outside of the interior space. The other kind of light sensor is called a “closed-loop” sensor. It generally is positioned on the ceiling, facing downward towards a horizontal work-surface. This sensor reads the light reflected from the horizontal work-surface. As the lights dim or brighten in response to a signal generated by the sensor, the system is adjusted to maintain a desired lighting level.
For interior spaces having one zone of lighting, the aforementioned closed-loop system is adequate. Within a closed loop system, one sensor, such as a photocell, couples to a dimmable control unit to control a multiple number of attached electrical loads, such as internal light sources, within one zone. In this zone, all internal light sources are dimmed at the same pre-determined rate of change in response to an increase or decrease in ambient light.
Adjusting all the internal light sources at the same rate is acceptable given the assumption that the external light sources affect every area of the internal space in the same way at all times of the day. However, for interior spaces that have, for example, windows along one side of the wall, the areas closest to the windows receive a higher amount of light than areas further from the windows. In such cases, a daylight harvesting scheme will require more than one zone, each having a number of internal light sources, wherein the rate for dimming the internal light sources within each zone differs. There, however, is no known closed loop system that is able to control lighting sources in multiple zones.
Open loop systems, however, may be used in the implementation of daylight harvesting for an interior space having multiple zones. Open loop systems include a light system for a specific interior space, a light control circuit or sensor and an external source of light. As mentioned above, the light control circuit is placed in a location inside or outside of the specific interior space. The light control circuit measures the external source of light. This measurement is fed back into the system to control the interior light sources, whereby, an outside source alone, i.e., the sun, controls the system output. The sun, in effect, acts as a potentiometer controlling the lighting control system. This type of system, however, suffers from less accurate control than closed loop systems because of seasonal and weather changes.
Thus, a need exists for a multi-zone daylight harvesting method and apparatus having a closed loop system that uses a single photocell or sensor to control a plurality of light sources in a plurality of zones.
The present invention is directed to overcoming, or at least reducing the effects of one or more of the problems set forth above.